It is after hours at Lister's Pool Room in Chicago, and once more pool shark Jesse Cardiff (Jack Klugman) is alone, polishing his pool game. Jesse bitterly muses that he would be considered the greatest pool player of all time, if it were not for the memory of the late Fats Brown overshadowing him. "I'd give anything, anything to play him one game!" he declares aloud.

"At your service!" comes a sudden voice from the corner of the room. It is indeed James Howard Brown (Jonathan Winters)—"known to [his] friends as Fats"—who has been dead for 15 years, but who has come from the afterlife to answer Jesse's challenge. Fats tells Jesse it is time for him to put his money where his mouth is and play a game of pool to see who the best truly is. But Fats ups the stakes: If Jesse wins, he will indeed be acknowledged as the greatest. If he loses, it means his life.

Jesse is undaunted, and the ultimate high-stakes pool game begins. All throughout the game, Fats laments that Jesse has done nothing with his life but play pool. Jesse ignores Fats, convinced that he is just trying to distract him from the game. When it comes down to one final, easy shot for Jesse to win the title, Fats warns him that he does not understand the burdens that come with being the best ever. Jesse ignores him and sinks the shot. He exults in his victory; he is now the best ever.

Fats's only response is to thank Jesse for beating him. Jesse is angered, declaring that Fats is a sore loser. Only years later, after he has died, does Jesse finally realize what Fats was warning him of. Jesse, as the new pool champion, is obliged to spend his afterlife defending his title, going from pool room to lonely pool room (his next destination: Mason's Pool Hall, Sandusky, Ohio), to play against challenger after challenger, just as Fats had to do until he finally lost. Meanwhile, Fats, finally free of his obligations as champion, has gone fishing.

Mr. Jesse Cardiff, who became a legend by beating one, but who has found out after his funeral that being the best of anything carries with it a special obligation to keep on proving it. Mr. Fats Brown, on the other hand, having relinquished the champion's mantle, has gone fishing. These are the ground rules in the Twilight Zone.

Johnson's script originally featured an ending in which Jesse loses the game and yet finds himself still alive. Seeing this, Fats explains that he will die "as all second raters die: you'll be buried and forgotten without me touching you. If you'd beaten me, you'd have lived forever."